DIAbatic influences on Mesoscale structures in ExTratropical storms

High winds and heavy rain are familiar features of the weather in north-west Europe. Forecasting these events accurately is of the highest priority for the national weather services, and indeed forecasts have improved considerably in recent years. But there is more to do – in particular, larger-scale storm systems that may be well forecast typically contain localised regions of particularly severe weather. These smaller regions, where much of the storm damage is often concentrated, are much more difficult to forecast than the storm itself, especially more than a day ahead.

At the heart of DIAMET is the way in which local heating and cooling affects the dynamics of storms. For example, as warm moist air ascends in a storm, it forms clouds of water droplets and then ice particles. Rain (and snow) forms through the growth and aggregation of these particles. But as water changes phase from vapour to water to ice, the heat energy that went into evaporating the vapour from the ocean surface is released – changing the temperature of the air. This sets up temperature gradients in the atmosphere that can have profound consequences for the development of storms. Capturing correctly the way the different particles behave, and the feedback between these processes and the storm itself, is a challenge that requires both state-of-the art weather forecasting models and detailed measurements.

The main tool that DIAMET is using for the detailed measurements is a specially-instrumented research aircraft operated by NERC and the Met Office. This aircraft, a BAe 146, can fly into the storms, making detailed measurements of the temperature, humidity and wind distribution as well as the cloud and precipitation particles. When combined with satellite and ground-based radar measurements, DIAMET scientists gain a powerful insight into exactly what is happening inside the storms.

To interpret these measurements DIAMET will use state-of-the-art numerical weather prediction models, in particular the Met Office's Unified Model. Models are used to provide detailed simulations of the observed storms, and our aim is to improve those simulations. We plan to do this in a number of ways: improving the way the model represents detailed processes like the freezing of cloud droplets, convection that starts well above the Earth's surface, and the input of heat and moisture from the ocean; improving the way detailed observations are used to initialise a model forecast; and helping to distinguish which of many possible model forecasts are likely to be correct.

DIAMET has conducted two aircraft campaigns already, in September and November-December 2011, and plans two more in May and July 2012. This will allow the team to investigate a range of weather events in different seasons. DIAMET got off to very good start in September, capturing several weather systems in the west and north of the UK. One of these in particular gave a prolonged period of heavy rain in the South-West and Midlands which proved particularly difficult to forecast accurately. Our good fortune continued in November-December with six more IOPs during a spell of particularly stormy weather. The highlight was our sampling of the very high winds over Scotland in Cyclone Friedhelm on December 8th.